2-Alkoxytetrahydrofurans and their derivatives are useful intermediates in the production of 1,4-diols and other valuable chemicals. See the co-pending application of William Edward Smith, Ser. No. 806,074, filed June 13, 1977.
A new process for the production of 2-alkoxytetrahydrofurans and their derivatives by an oxo tetrahydrofuranylation reaction comprising the hydroformylation of allylic alcohols in the presence of saturated alcohols and rhodium carbonyl catalysts is described in applicant's co-pending application Ser. No. 806,703, filed June 13, 1977, and incorporated herein by reference. The process was discovered when a triphenylarsine-modified rhodium carbonyl catalyst was used in the conventional hydroformylation of allyl alcohol -- some 2-n-propoxytetrahydrofuran was found among the reaction products. As is described in the latter application, when solvent or coreactant quantities of methanol, n-propanol, etc., are present, good to excellent yields of the corresponding alkoxytetrahydrofurans are produced depending on the allylic alcohol substrate.
The role of the triphenylarsine catalyst component in the initial work appears to have been more than incidental. Triphenylarsine appears to form a relatively weak complex with the rhodium carbonyl hydride catalyst intermediate, e.g., EQU H Rh(CO).sub.3 L
wherein L is As(C.sub.6 H.sub.5).sub.3. This complex is apparently quite acidic, rather than hydridic, as is indicated by the ready formation of 1,1-dimethoxypropane from methanol and propionaldehyde in a side reaction.
In contrast, the triphenylphosphine/rhodium carbonyl catalyst system, also described in the second-mentioned application, does not seem to exhibit the same "acidic" character. While triarylphosphine complexed catalysts are excellent hydroformylation catalysts, they do have the disadvantage of not favoring the conversion of the intermediate 2-hydroxy-tetrahydrofuran or substituted-tetrahydrofuran to the corresponding 2-alkoxytetrahydrofurans without extended and/or vigorous heating.
It has been discovered that the new oxo tetrahydrofuranylation process can be improved by the use of acids as co-catalysts for the rhodium carbonyl/tertiaryphosphine complex catalyst. The new rhodium carbonyl/tertiaryphosphine/acid co-catalyst system is superior to rhodium carbonyl/tertiaryphosphine with respect to the yield of the more stable 2-alkoxy product, a finding which has important implications relative to production of 1,4-butanediol from allyl alcohol via 2-methoxytetrahydrofuran or other 2-alkoxytetrahydrofurans.
Merely by way of illustration, o-phthalic acid serves the co-catalytic function well without promoting undesired side reactions. It is strong enough as an acid to promote the conversion of the intermediate 2-hydroxytetrahydrofuran to the 2-alkoxy product, but does not significantly decompose the other initial oxo product, 3-hydroxy-2-methylpropionaldehyde. Moveover, the fact that the acid co-catalyst does not interfere with the hydroformylation reaction itself is entirely unexpected. The hydrolysis-hydrogenation step according to the present discovery produces the potentially valuable 2-methyl-1,3-propanediol as the major byproduct (about 30:70 vs. 1,4-butanediol), with only minor amounts of n-propanol and isobutanol formed. Such transformations are illustrated in the following pathway: ##STR1##
In a controlled comparison of the hydroformylation of allyl alcohol in the presence of methanol with a rhodium carbonyl/triphenylphosphine catalyst per se, and also complexed with o-phthalic acid co-catalyst according to this invention, the ratio of 2-methoxytetrahydrofuran to 2-hydroxytetrahydrofuran at 115.degree. C. was markedly and favorably altered by the co-catalyst from 6:94, without, to 97.3, with.